In recent years, there are increasingly accelerated higher integration, higher functionality, and higher density mounting of semiconductors extensively used in electronic equipments, communication instruments, and personal computers or the like. This demands better properties and higher reliability of laminates for semiconductor plastic packages. From a growing interest in environmental problems, a laminate having heat resistance high enough to be applicable to a reflow process at high temperatures is demanded in order to use lead-free solders.
In recent years, a reduction in coefficient of thermal expansion in a plane direction of laminates is strongly demanded. When the difference in coefficient of thermal expansion between a semiconductor element and a printed wiring board for a semiconductor plastic package is large, warpage occurs in the semiconductor plastic package due to the difference in coefficient of thermal expansion upon exposure to thermal shock, which may cause poor connection between the semiconductor element and the printed wiring board for a semiconductor plastic package or between the semiconductor plastic package and the printed wiring board mounted. An increase in heat value caused by the high-density of the printed wiring board has demanded a very low coefficient of thermal expansion in the plane direction.
A method for filling a resin composition as a raw material with an inorganic filler is known as a general method for reducing the coefficient of thermal expansion in the plane direction of the laminate. A large amount of the inorganic filler to be filled reduces the coefficient of thermal expansion. However, this poses a problem that the moldability of the laminate is lowered.
The incorporation of an organic filler having rubber elasticity in a resin composition is known as other technique for a reduction in thermal expansion in a plane direction (for example, Patent Literatures 1 to 5 or the like). However, such a laminate containing the organic filler has a drawback of lowered flame retardance.
In order to solve this problem, use of a silicone rubber powder instead of an organic filler is proposed (Patent Literature 6). However, even the proposed material has not yet reached a sufficiently low coefficient of thermal expansion demanded in recent years. A material having a lower coefficient of thermal expansion has been desired.